The present invention relates to nondispersive infrared gas analyzers, and is directed more particularly to gas analyzers in which infrared detectors of differing types cooperate to provide gas concentration readings which are corrected for the effects of a variety of different disturbing influences.
In situations in which the concentration of one component of a sample gas is to be determined, it has long been the practice to make the determination by passing infrared radiation through the sample gas and measuring the infrared radiation absorbed thereby. By restricting the wavelength range to those bands in which the component of interest has distinctive absorption characteristics, the instrument may be made selectively responsive to that particular component of interest.
Instruments of the above-described type may be classified on the basis of the infrared detection devices and schemes employed therein. One type of infrared analyzer includes analyzers in which infrared absorption is measured through the use of black body detectors. These detectors are typically solid-state devices such as thermistor flakes, thermocouples, pyroelectric detectors and the like which are responsive only to the total quantity of infrared radiation to which they are exposed. In analyzers that use this type of detector, the detector is usually illuminated through a narrow band pass filter which causes the detector to respond only to a band of wavelengths that contains absorption bands that are characteristic of the component of interest. One analyzer of this type is shown, for example, in U.S. Pat. No. 3,920,993, issued on Nov. 8, 1975, in the name of C. N. Cederstrand, one of the inventors named in the present application. Another analyzer of this type is shown in U.S. Pat. No. 3,539,804, issued on Nov. 10, 1970, in the name of A. C. Billetdeaux et al. Still another analyzer of this type is shown and described in copending patent application Ser. No. 98,469, entitled "Split Detector", filed on Nov. 29, 1979, in the name of C. N. Cederstrand et al.
Another type of infrared gas analyzer includes analyzers in which detection is accomplished by means of pneumatic detectors, also known as Luft-type detectors. These detectors are typically gas filled, pressure-responsive devices that respond only to the quantity of infrared radiation that is absorbed at the specific wavelengths at which the filling gas has absorption lines. In analyzers that use this type of detector, measurements of the concentration of the component of interest are based on the difference in the pressure inside the detector when a sample gas of unknown composition is present in the sample cell and the pressure inside the detector when a zero gas is present in the sample cell.
Included among analyzers that use pneumatic detectors are analyzers which utilize two or more detectors to reduce or eliminate the effect of some disturbing influence such as ambient mechanical vibrations. An analyzer of the type having a plurality of pneumatic detectors that are arranged to correct for the effects of mechanical vibration and shock is described in U.S. Pat. No. 3,770,974, issued on Nov. 6, 1973, in the name of G. H. Fertig. An analyzer of the type having a plurality of pneumatic detectors that are arranged to compensate for the effect of source intensity variations is shown and described in U.S. Pat. No. 3,130,302, issued on Apr. 21, 1964, in the names of M. D. Liston et al. An analyzer of the latter type which is also said to compensate for the effect of interfering compounds in the sample gas is described in U.S. Pat. No. 2,924,713, issued on Feb. 9, 1960, in the name of M. D. Liston.
Other patents describing analyzers having various types and configurations of pneumatic detectors are shown in U.S. Pat. No. 2,957,076, issued on Oct. 18, 1960, in the name of S. A. Francis; U.S. Pat. No. 3,898,462, issued on Aug. 5, 1975 in the name of Ishida et al; and U.S. Pat. No. 4,156,812, issued on May 29, 1979 in the name of J. Staab.
Prior to the present invention there had also been developed infrared gas analyzers of the type having both black body and pneumatic detectors. In copending application Ser. No. 117,175, now U.S. Pat. No. 4,355,233, entitled "Method and Apparatus for Negating Measurement Effects of Interferent Gases in Nondispersive Infrared Analyzers", filed on Jan. 31, 1980 in the name of C. N. Cederstrand et al, there is described an analyzer in which a plurality of black body detectors having respective band pass filters are positioned between a sample cell and a pneumatic detector. In the latter analyzer, however, the black body detectors are dedicated to developing signals for use in cancelling the effect of respective interfering compounds.
A common feature of error cancellation schemes that utilize pneumatic detectors is that they include at least two pneumatic detectors that are connected in optical series with one another. While the latter configuration is effective in cancelling of the effect of disturbing influences, such as mechanical vibration and shock, that affect similar detectors in a similar way, it is ineffective in cancelling the effect of disturbing influences, such as source intensity variations and the presence of interfering compounds, that affect similar detectors in different ways. This is because the downstream detector is exposed to infrared radiation having a wavelength distribution which has been altered by the selective absorption of radiation by the upstream detector through which it is illuminated. As a result, while the pneumatic detectors themselves may be similar, the fact that they are exposed to beams of radiation having differing wavelength distributions prevents them from producing signals having source and interferent-related error components that may be canceled out in the same way that vibration-related error components may be canceled out.
More generally, the problem with analyzers having like types of detectors is that the need to simultaneously correct for different types of disturbing influences imposes conflicting correction requirements. For disturbing influences such as mechanical vibrations which affect like detectors in the same way, the cascading of like detectors facilitates the correction process. For disturbing influences such as source intensity variations and interfering compounds which affect like detectors differently, on the other hand, the cascading of like detectors hinders the correction process.
If, for example, the output signals from like types of detectors are combined so that vibration-related error components cancel, then the effect of source-related and interferent-related error components do not cancel. Conversely, if like types of detectors are modified to allow the correction of source and interferent related errors, then the ability to compensate for vibration related errors is adversely affected. Thus, prior to the present invention, analyzers that used a plurality of similar infrared detectors could not compensate for different types of disturbing influences simultaneously.
While the analyzer described in the last-mentioned copending application is not subject to the above-described problem, its usefulness is limited in a different way. With one or more black body detectors positioned upstream of the pneumatic detector, there is necessarily a blockage of the radiation that would otherwise fall on the pneumatic detector. This problem is compounded if, as is not unlikely, two upstream black body detectors are used, one to monitor source intensity variations and a second to monitor interferent concentration variations. Significantly, if the upstream black body detectors are made small enough to reduce such problems, the strengths of their output signals and their signal-to-noise ratios becomes unacceptably low. Thus, difficult performance tradeoffs may be necessary.